In conventional moire contouring techniques an amplitude grating such as a Ronchi ruling is placed close to an object or test surface whose contour is to be determined. The test surface is illuminated through a grating surface by radiation directed at an angle .theta. to a normal of the grating surface. Observation takes place along a normal, and the moire pattern generated between the projected pattern on the object and the grating represents an instantaneous display of the elevation contours. Each of the fringes represents areas of equal elevation or height with reference to a datum plane of the surface. Typically these contours are digitized using complex data manipulation, algorithms, and large digital computers. Areas between the fringes must be ascertained by interpolation using even more complex data manipulation algorithms. This approach thus requires large, very powerful computing equipment which is expensive and in spite of its size and speed requires much time to complete the computations. These machines must be specially constructed or specially programmed to perform the analysis. The measurement is also time consuming and may take a minute or more. The extended time required for measurement leads to additional problems: vibrations taking place in the area of the machine interfere with the operations.
In one technique, the shadows on the test surface are formed using a projection system. The shadows or patterns on the object can be formed by either projecting the image of a Ronchi ruling or similar sinusoidal pattern onto the test surface or by interfering two coherent plane waves on the test surface. The pattern on the test surface is in turn directed through another Ronchi grading or sinusoidal pattern, the same as the first, and the resulting moire fringes represent the elevation or height contours which can then be processed in the same way. However, since the contour intervals or fringes are a function of the period of the projected pattern, the resolution of the moire contouring method depends upon the spatial frequency of the projected pattern. In order to increase resolution, the projection of the imaging systems must be low F-number systems. But this limits the elevation range, since as the F-number of an optical system is lowered its depth of field is reduced exponentially according to the second power. Attempts to improve the resolution while maintaining better depth of field and elevation range have met with indifferent success.